This invention is directed to the field of materials fabrication and specifically, to the field of materials fabrication involving the joining of materials through the use of controlled atmosphere bonding.
One of the most significant inventions of the twentieth century is that of the microprocessor. The ability to provide intelligence on a chip has fostered countless products ranging from cellular telephones to portable life support systems to hand held video games. While the processing ability of these computers on a chip has increased dramatically since their introduction, perhaps even more significant is the fact that as the power of these chips has continued to increase, the cost of these devices has decreased. Indeed, the processing power which once would have cost hundreds of thousands of dollars can now be purchased for just a few dollars. Because of their favorable cost to performance ratio, microprocessors are now nearly ubiquitous in most developed countries. As in most areas of technology, the companion technologies which must be employed to produce these devices such as the microprocessor have advanced as well.
Because of the complexity and high density of electronic components in these devices, coupled with the fine geometries at which they must be fabricated, it is not surprising that their production processes must be carefully controlled. By carefully controlling each step of the complex manufacturing processes, engineers have been able to continuously improve the state of the art, thereby providing dramatic improvements not only in the function of the devices being produced but also in the yield of the processes used to produce these devices. The processes of Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) are frequently employed to produce exotic or difficult to create materials on substrates. Many semiconductor fabrication processes such as chemical vapor deposition require that the substrate on which materials are to be deposited be first thoroughly cleaned and subsequently heated prior to and during the actual deposition process. The heating process must be controlled precisely since variations in temperature of the substrate can result in variations of the properties of the thin films to be deposited or, indeed whether deposition occurs in the first place. Additionally, if the substrate is not heated and ultimately, cooled evenly and in a precise manner, warping may occur which can result in permanent deformation of the substrate, rendering it unsuitable for the intended purpose.
A variety of means may be employed in the heating of materials such as substrates. Firstly, if the substrate is electrically conductive, it may be directly heated. An electric current may be passed through the substrate thereby causing it to be heated proportionately to the voltage presented to and current passing through the substrate. Many of the materials used for fabrication of micro sensors and integrated circuits for example, are glasses, ceramics or, in the case of semiconductors, silicones. These materials are at best, poor electrical conductors so this heating means is seldom employed when processing these materials.
A heated plasma or an energy beam such as a laser or electron beam may be used to heat the object. These processes are unfortunately, rather complicated and quite costly. Additionally it is difficult to control these energy sources in a manner which will promote even heating of the entire object to be heated. Further, the energy fields induced in the substrate by these methods may damage the substrate or coatings or circuit elements (if any) thereon.
A conductive heating process may be employed which involves placing the item to be heated directly on a heated plate whereby heat is transferred from the heated plate to the object to be heated. Traditional hot plate systems are however, notorious for being anisothermal, thereby creating hot spots in the object being heated which can lead to mechanical deformation or to other process failure.
Conductive heating may also be accomplished by using a suitable gas to transfer heat from a heat source to the object to be heated. Ovens using this process are often referred to as atmospheric ovens and may employ inert, oxidizing or reducing atmospheres as may be beneficial to the process. Mechanically stirring the heated gas serves to make this type of heating process more or less isothermal and minimizes temperature variations which may be induced by thermal convection or through the use of more or less point source heaters. This process is of course not possible in the event that the object to be heated must be processed in a vacuum. Since chemical vapor deposition often takes place at reduced pressures of for example, in the range of 0.1 torr 1 atmosphere, the process of conductive heating employing gasses may not be practical in some of these applications.
Radiative heating may be used to heat the object whereby infrared energy is transferred by radiation from a heated source to the object, whether the object is in vacuum or in an atmosphere. Radiative heating offers an advantage over direct conductive heating in that the object to be heated need not be in contact with the heat source. Eliminating this contact also eliminates surface defects on the object to be heated which can be created when contaminants on the heat source are transferred to the object to be heated. Additionally, if temperatures and contact forces between the heat source and the object to be heated are sufficiently high, diffusion bonding or welding may occur, thereby bonding the object which is to be heated to the heat source and potentially ruining the process part. Ideally, a highly emissive, heated, isothermal plate is used as a radiation source to transfer heat to the object to be heated. Desirable qualities of this plate include flatness and the ability to accurately control thermal radiation from the surface of the plate. The task of producing the desired radiation source which is capable of providing a nearly constant thermal emission over the entire surface is formidable. This task is further complicated when the object and hence, the required radiating plate are of significant size. Additionally, some means for precisely heating and preferably also, cooling the plate must be accomplished. These same critical issues also apply to the previously described process of conductive heating. Copper for example, exhibits a high degree of thermal conductivity but relatively low resistance to bending moments, particularly at elevated temperatures. Additionally, copper is a relatively active metal which may interact negatively with other materials in the process and/or contaminate the operating environment. Stainless steels offer mechanical strength and are relatively inert, but exhibit poor thermal conductivity. Both copper and stainless steel have relatively low emissivity in their natural state, making them inefficient radiators of thermal energy. It can be seen then, that using either of these materials alone fails to provide for an effective heat source for either radiative or conductive heating of planar materials.
U.S. Pat. No. 5,192,623 discloses laminated structural panels and a method of producing them. This patent teaches to the production of structural panels in which at least one of the panel members is perforated so that when laminated between two imperforate panels and brazed in a high vacuum environment, that a substantial vacuum will be formed and maintained between the imperforate panels in the void space created between the panels and the perforations in said perforated plate. Such a panel containing these vacuum pockets is useful as an insulator of temperature and sound. As such, the panel described in U.S. Pat. No. 5,192,623 would not be suitable as a source of isothermal radiation, nor cooling, nor fluid transport.
U.S. Pat. No. 4,359,181 provides for a dip brazed, laminated heat transfer surface. This patent discloses a process to produce an improved fluid to fluid heat exchanger through the use of laminated expanded metal members. No claim is made with respect to using this invention as a source of isothermal radiation or conductive heating.
A relatively common type of heat exchanger is described in U.S. Pat. No. 4,592,415. The thin, heat exchanger produced by the process of U.S. Pat. No. 4,592,415 comprises two photochemically etched plates which are sealingly joined together so that a fluid passage is created between said plates. The process is not well suited to the production of relatively large planar members which require a high degree of stiffness and flatness.
A laminated plate header for a refrigeration system and method for making same is taught in U.S. Pat. No. 5,242,016. This invention is intended to selectively distribute refrigerant from a header inlet to a heat exchanger. Brazing is mentioned as a possible means of assembling the various components which comprise the system. The device of this invention is directed only at refrigeration systems and particularly, at the distribution of refrigerant to the core units of a heat exchanger. No mention of use as a flat, heated panel is made.
U.S. Pat. No. 4,029,254 describes a method of diffusion bonding and brazing of materials as may be used to produce jet engine housing ducts. The method described uses a process whereby the materials to be joined are placed more or less coaxially inside of a cylindrical container. A male plug is inserted through the center of the innermost member to be joined, which plug has a coefficient of thermal expansion which is greater than the material it faces and the core material. As the entire assembly is heated, this differential rate of thermal expansion causes the male plug to exert force against the interior of the assembly so that intimate contact between the parts to be joined may be achieved. An arrangement of ceramic or other pellets controls the ultimate forces holding the parts together during the brazing or diffusion process. The process claims are limited to the use of honeycomb core materials and no mention is made of using the means of this invention to produce flat, radiative panels, or cooling, chucks, or fluid transmission.
A process for simultaneously forming and brazing of titanium or aluminum components is taught in U.S. Pat. No. 5,821,506. The method described in this invention provides for an inductively heated thermal process which first forms metal parts through superplastic deformation and then, in the same process, brazes these parts in a retort. The interior faces of the retort are coated with a release agent, such as boron nitride so that the retort does not become bonded to the parts being processed. After the process is completed, the retort is cut away. The process is directed at the production of structural aerospace assemblies.
Similarly, U.S. Pat. No. 5,420,400 teaches a sequential process of forming and brazing using a disposable retort.
A combined cycle for forming, annealing, (particularly annealing of titanium) and brazing is described in U.S. Pat. No. 5,914,064. Induction heating through ceramic dies provides for an efficient heat transfer process, however, the retort used in the process described in this patent is deliberately not bonded to the parts to be processed and the retort is completely discarded from the part after processing. Additionally, there is no teaching to the desirable formation of an oxide layer to enhance thermal radiation by increased surface emissivity or even to the use of the formed part to serve as a heating or cooling member.
A method specific to making titanium aluminide metallic sandwich structures is described in U.S. Pat. No. 5,118,026. A diffusion bonding process is combined with a superplastic forming process in order that sandwich structures of definite geometry may be constructed. A disposable retort is used here to maintain an oxygen and contaminant-free environment in order that diffusion bonding may take place. Once again, this patent teaches to the fabrication of a structural member without mention of use of such member as a device to be used to impart heating or cooling to other objects. Additionally, there is no mention of the desirable formation of an oxide layer, in fact the text teaches away from the formation of oxides.
What is lacking therefore is a means to fabricate a system whereby objects such as, for example, semiconductor materials or CVD substrates can be heated in a precisely controlled process, whether that process takes place in an atmosphere or vacuum. Imperative in the design of such a system is high thermal conductivity in the heated plate, high resistance to bending moments in the heated plate and high emissivity on the surface of the heated plate. Additionally, it may be desirable that the system offer the capability of rapid cooling, since many processes require accelerated cooling to quench or otherwise thermally process the materials being produced. Gas transmission capability is also desirable for use as a heated chuck.
If a means could be devised to produce planar elements which possessed the desired qualities previously discussed, it would greatly enhance the ability to thermally process materials whether that process occurred in vacuum or in an atmosphere. Said means would be invaluable for the production of equipment to be used in the precise thermal processing of for example, sheets of glass or semiconductor substrates. Such a system could of course also be used to produce planar or other forms of elements which would undoubtedly find application in for example, chemical or biological material processing. The instant invention then provides for a means and economical process to produce a multi-laminar element which has high thermal conductivity throughout its planar surface, is robust and resistant to flexure or bending, has a high degree of flatness over its surface as may be desired, exhibits high surface emissivity, is essentially inert with respect to corrosive environments and may be precisely heated and cooled thereby permitting the plate to cause heating and cooling in other materials which may be in contact with or proximal to said element.
The instant invention provides for a means and process to produce a high compliance multi-laminar element or structure which finds application in the processing of materials such as semiconductor substrates, or chemical or biological materials.
Further, it is an object of this invention to provide for a means of producing multi-laminar elements which exhibit high thermal conductivity.
Still another object of the instant invention is to provide for a means to produce multi-laminar elements which have surfaces exhibiting high thermal emissivity.
It is a further object of this invention to disclose a means which may be employed to produce multi-laminar elements which are highly resistant to flexure and bending moments.
Yet another object of the instant invention is to provide for a process which may be used to produce multi-laminar elements which may be heated by means of internal electric heating elements.
Yet still another objective of this invention is to provide for a system which permits the production of multi-laminar elements which may be heated and/or cooled through the use of hot or cold gasses or other fluids which may traverse passages within said planar elements.
Additionally it is an object of the instant invention to provide for a means to produce multi-laminar elements which are relatively chemically inert as may be required in the processing of semiconductor and similar materials.
Another object of this invention is to provide for a means by which materials may be joined whereby a portion of said materials interact with a differential gas pressure thereby creating a condition of dynamic loading to enhance the joining process.
Still another object of the instant invention is to teach a means whereby, multi-laminar elements may be produced, said elements having a high degree of flatness.
Another object of this invention is to provide for a means whereby a very high degree of thermal conductivity may be achieved between two planar materials which are brazed together.
Further, it is an object of this invention to provide for a means whereby the process of transient liquid phase diffusion bonding is employed to assemble components of multi-laminar systems.
Still further, it is an object of this invention to provide for a means whereby the process of diffusion bonding known as transient liquid phase diffusion bonding is employed to assemble components of multi-laminar systems.
Yet another object of this invention is to provide a means to mark and henceforth precisely identify hidden positional reference points so that precise automated machining processes may be accomplished.
Another object of the instant invention is to provide for a means of dynamic loading of a plurality of materials whereby members using in the loading process become a useful part of the finished assembly.
Yet still another object of this invention is to provide for a process whereby the emissivity of an object may be beneficially enhanced while simultaneously joining materials.
Further, it is an object of this invention to provide for an efficient means whereby elements may be precisely aligned and protected during a bonding process.
Still another object of the instant invention is to provide a means of economically joining materials.
It is a further object of this invention to provide a process which is economical in that said process combines several processing steps into one-operation.
It is a further object of this invention to provide a process which is economical in that said process utilizes relatively inexpensive brazing alloys.
Yet still another object of this invention is to provide for a process whereby the required quantity of braze alloy for any given brazing operation is minimized by controlling flow of said alloy and thereby promoting its efficient use.
Further, it is an object of the instant invention to provide for a method of integral containment which produces minimal contaminants to the brazing furnace by the process of the instant invention.
Yet further, it is an object of this invention to provide for a single process whereby the dynamic loading, bonding, control of alloy flow and formation of a desirable oxide surface layer, or alternately, an ultra clean surface may be accomplished simultaneously.
Additionally it is an object of this invention to provide for a process of bonding components whereby said components may be formed either prior to or after said bonding operation.
Still another object of the instant invention is to provide for a bonding process whereby a high vacuum source is advantageously distributed about the periphery of a centrally located core plate.
Other objectives and advantages of this invention will become apparent from the accompanying descriptions taken in conjunction with the accompanying drawings wherein set forth, by way of illustration and example, are certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. It will be readily appreciated by those skilled in the art that the use of the process described in the instant invention is highly effective and useful.